This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application Nos. 2004-075900 and 2005-040675 filed in Japan on Mar. 17, 2004 and Feb. 17, 2005, respectively, the entire contents of which are hereby incorporated by reference.
The present invention relates to a bidirectional photothyristor chip, a light-triggered coupler with use of the same, and a solid state relay (hereinafter abbreviated to SSR) with use of the light-triggered coupler.
Conventionally, there has been a bidirectional photothyristor with the structure shown in FIG. 35 to FIG. 37. It is to be noted that FIG. 35 is a plane view, FIG. 36 is a cross sectional view taken along an arrow line A-A′ of FIG. 35, and FIG. 37 is an equivalent circuit diagram. The bidirectional photothyristor chip 4 is composed of, for example, a CH (channel) 1 photothyristor and a CH2 photothyristor formed on an N-type silicon substrate 1. Such a bidirectional photothyristor chip 4 is widely used for a light-triggered coupler which produces a gate trigger signal through photoirradiation to perform on-off control over the SSR.
It is to be noted that there are shown anode diffusion regions (P type) 5, 5′, P-gate diffusion regions (P type) 6, 6′, cathode diffusion regions (n type) 7, 7′, gate resistances 8, 8′, Al electrodes 9, 9′ and an Al electrode 10. It is to be noted that an electrode T2 is formed immediately above the Al electrode and is connected to the anode diffusion region 5 and the cathode diffusion region 7 through the Al electrode 9. Similarly, an electrode T1 is formed immediately above the Al electrode 9′ and is connected to the anode diffusion region 5′ and the cathode diffusion region 7′ through the Al electrode 9′. A PNPN section that constitutes a photothyristor 2 of CH1 in FIG. 37 is formed extending from the anode diffusion region 5′ on the right-hand side toward the cathode diffusion region 7 on the left-hand side as viewed in the drawing. Moreover, a PNPN section that constitutes a photothyristor 3 of CH2 in FIG. 3 is formed extending from the anode diffusion region 5 on the left-hand side toward the cathode diffusion region 7′ on the right-hand side as viewed in the drawing.
FIG. 36 is a cross sectional view of the N-type silicon substrate 1 showing a passivation structure in the bidirectional photothyristor. An SiO2 film 15 is formed over the cathode diffusion region 7 on the left-hand side of the Al interconnection 10 to the anode diffusion region 5′ on the right-hand side of the Al interconnection 10 on the N-type silicon substrate 1. Further, an oxygen-doped semi-insulating polycrystalline silicon film 16 is formed on the SiO2 film 15, and further on the oxygen-doped semi-insulating polycrystalline silicon film 16, an SiN film 17 is formed by the chemical vapor deposition method. On the left-hand side, the Al electrode 9 is formed over the SiN film 17 to the P-gate diffusion region 6, and is connected to the electrode T2. On the right-hand side, the Al electrode 9′ is formed over the SiN film 17 to the anode diffusion region 5′, and is connected to the electrode T1. Further, as shown in FIG. 35, the Al interconnection 10 for separating the left-hand side and the right-hand side of the bidirectional photothyristor as viewed in the drawing is formed on the SiN film 17 over its entire width and is connected to the N-type silicon substrate 1. Thus, both the ends and the center portion of the oxygen-doped polycrystalline silicon film 16 are brought into contact with the Al electrodes 9, 9′, 10 to form a potential gradient between the Al electrodes 9, 9′ and the Al electrode 10 for alleviating concentration of electric fields on the Si—SiO2 interface. Thus, the field plate structure which advantageously offers high withstand voltages is provided. It is to be noted that reference numeral 18 denotes an N+ layer and reference numeral 19 denotes a depletion layer.
Generally, light-triggered couplers used with an alternating current operates as follows. That is, in FIG. 37, if the potential polarity is more positive on the electrode T1 side than on the electrode T2 side under the condition that alternating voltage higher than the on-state voltage (about 1.5 V) of the device is applied as a bias to between the electrode T1 and the electrodes T2, then an NPN transistor Q2 on the CH1 side is turned on when the bidirectional photothyristor 4 receives a light signal from an LED (Light-Emitting Diode) (unshown). As a result, a base current is extracted from a PNP transistor Q1 on the CH1 side, and the PNP transistor Q1 is turned on. Next, a base current is supplied to the NPN transistor Q2 on the CH1 side by a collector current of the PNP transistor Q1, and the PNPN section on the CH1 side is turned on by positive feedback, as a result of which an on-state current corresponding to the load of an AC circuit flows from the electrode T1 to the electrode T2. In that case, the positive feedback of the PNPN section does not occur on the CH2 side since the bias applying direction is reversed, and only a primary photoelectric current flows. In the next half cycle, if the potential polarity is more positive on the electrode T2 side than on the electrode T1 side, then the PNPN section on the CH2 side is turned on through positive feedback operation in entirely the same way as the above-mentioned case, and only the primary photoelectric current flows on the CH1 side.
Thus, when light is continually received from the LED, the bidirectional photothyristor 4 is turned on. When light is not emitted from the LED, the bidirectional photothyristor 4 is turned off at a holding current value (hereinafter referred to as IH). Thus, the switch function is fulfilled. It is to be noted that the prior art documents regarding the bidirectional photothyristor for use in the light-triggered coupler include, for example, JP H10-242449 A.
However, the conventional bidirectional photothyristor has a following problem. That is, increase in luminous sensitivity to have high sensitivity decreases a commutation characteristic that is noise resistance as well as a dv/dt characteristic which are contradictory to the luminous sensitivity. More particularly, the commutation characteristic and the dv/dt characteristic are in a trade-off relation with the luminous sensitivity, which are the most important design issue in terms of the performance of the bidirectional photothyristor. The dv/dt characteristic herein refers to “critical rate of rise of off-state voltage”, and the critical rate of rise of off-state voltage of not lower than 1000 V/μs is required for the bidirectional photothyristor to correctly function as a device.
It is to be noted that in view of equipment in use, high sensitivity leads to control with smaller current to provide advantages that lower power consumption is achieved and that direct driving from a micro computer and the like is available. This high sensitivity is an important characteristic strongly desired by users.
Description is given of the commutation characteristic. With regard to the commutation characteristic in the case of normal operation, as shown in FIG. 38 (vertical cross sectional view showing the entire region including a region along the line A-A′ of FIG. 35), if the incidence of light disappears in a half cycle period of the alternating current during which the CH1 is on, then the on-state continues due to the current holding property of the PNPN section during this half cycle period. Then, if a shift to the next half cycle occurs as shown in FIG. 39 (vertical cross sectional view showing the entire region including the anode diffusion region 5 and the cathode diffusion region 7′ in FIG. 35), then the CH2 is not turned on unless there is incident light. However, if an inductive load exists in the AC circuit that is subjected to switching, then the phase of the on-state voltage is delayed relatively to the phase of the AC voltage applied to between the electrode T1 and the electrode T2. Therefore, an AC voltage of the inverted phase has already been applied to between the electrode T1 and the electrode T2 at the point of time when the CH1 is turned off. Therefore, a voltage of the inverted phase exhibiting a steep rise is to be applied to the CH2 side at the point of time when the CH1 is turned off.
Because of this reason, holes 11, which remain in the N-type silicon substrate 1 of the bidirectional photothyristor 4, move to the P-gate diffusion region 6′ on the side of the photothyristor 3 as indicated by arrow (A) before disappearing to thereby turn on the PNP transistor Q4 on the CH2 side despite no incident light and to promote the positive feedback on the CH2 side, causing malfunction (commutation failure) that the CH2 is turned on.
More particularly, the “commutation characteristic” is the characteristic which expresses a maximum operating current value Icom that can be controlled without causing the commutation failure as described above. There is a trade-off correlation that higher sensitivity decreases the commutation characteristic, and therefore how to increase the commutation characteristic is an issue in pursuit of higher sensitivity.
Meanwhile, in order to prevent the commutation failure, all that is necessary to be done is to suppress the residual holes 11 in the N-type silicon substrate 1 from moving from the side of the photothyristor 2 to the P-gate diffusion region 6′ on the side of the photothyristor 3. However, in the conventional bidirectional photothyristor 4 having the structure shown in FIG. 35 to FIG. 37, its passivation structure as described above is the field plate structure in which, as shown in FIG. 36, a potential gradient is formed between the Al electrodes 9, 9′ and the Al electrode 10 to alleviate concentration of electric fields on the Si—SiO2 interface so that high withstand voltages are advantageously provided. However, such structure has no direct relation with improvement of the commutation characteristic and therefore fails to suppress the holes 11 which are generated on the side of the photothyristor 2 and remain in the N-type silicon substrate 1 from moving to the P gate diffusion region 6′ on the side of the photothyristor 3.
Description is now given of the critical rate of rise of off-state voltage dv/dt characteristic. When a voltage pulse with a steep rise is applied to between the anode diffusion regions 5, 5′ and the cathode diffusion regions 7, 7′, malfunction that the bidirectional photothyristor 4 is turned on without any light signals occurs. This is because a displacement current flows into the P-gate diffusion regions 6, 6′ which are intended to receive a light signal and the displacement current acts as a trigger current. This kind of malfunction tends to occur particularly in the high temperature condition. More particularly, a maximum rate of rise of voltage that does not cause the malfunction is a critical rate of rise of off-state voltage dv/dt. The critical rate of rise of off-state voltage dv/dt characteristic also has a trade-off correlation with the sensitivity that increase in sensitivity decreases the characteristic. More particularly, how to enhance the dv/dt characteristic is also an issue in pursuit of higher sensitivity.